1. Field of Invention
This invention relates to a method of preparing L- and D-5,5,5,5′,5′,5′-hexafluoroleucine and protected analogs thereof, including compounds having a protected amino functional group, compounds having a protected carboxy functional group, and compounds having protected amino and carboxy functional groups. These compounds are useful for the preparation of fluorous peptides and proteins.
2. Discussion
Hexafluoroleucine is of considerable interest as an extensively fluorinated analog of leucine that may be used as a building block in the design of fluorous proteins and peptides (E. N. G. Marsh, 7 Chem. Biol. R153 (2000)). Fluorocarbons have long been known for their chemical inertness, and their unique physicochemical properties have found industrial and medical uses as fire retardants, refrigerants, anesthetics and biologically inert polymers. The tendency of extensively fluorinated, or fluorous, organic molecules to partition into perfluorinated solvents has been exploited in organic synthesis to facilitate purification of products from reaction mixtures (I. T. Horvath, 31 Acc. Chem. Res. 641 (1998); Z. Y. Luo et al., 291 Science 1766 (2001)).
Recently, there has been much interest in whether the novel properties exhibited by fluorocarbon polymers can be exploited in the design of biological macromolecules. Extensively fluorinated analogs of hydrophobic amino acids, when substituted into proteins and peptides, pack into the hydrophobic core of the protein to produce proteins that combine novel physicochemical properties with biological activity (Y. Tang et al., 40 Biochemistry 2790 (2001)). Peptides designed to form dimeric coiled-coil structures based on the “leucine zipper” domain of the transcription factor GCN4, or de-novo designed sequences that incorporate (4R, 4S)-L-trifluoroleucine, (3R, 3S)-L-trifluorovaline or L-hexafluoroleucine, display increased stability, enhanced self association and stronger receptor-ligand binding than their non-fluorinated counterparts.
Hexafluoroleucine (hFLeu) is a highly fluorinated analog of leucine, an amino acid that plays an important role in the folding of many proteins. A racemic synthesis of hexafluoroleucine was reported in 1968, and the first chiral synthesis of this amino acid was reported in 1998 (J. Lazar & W. A. Sheppard, 11 J. Med. Chem. 138 (1968); C. Zhang et al., 81 Helv. Chem. Acta 174 (1998)). The highest enantiomeric purity achieved in this prior work was 81% e.e. and this weakness substantially limits the value of the route for the production of hFLeu for peptide synthesis. More recently, Kumar and coworkers (X. Xing et al., 3 Org. Lett. 1285 (2001)) reported a ten-step synthesis of L-hFLeu from D-serine. This synthesis was reported to provide L-hFLeu in >51% yield and >99% e.e. from Garner's aldehyde, which in turn is prepared from D-Serine in five steps. This synthesis is long, difficult to reproduce, and provides material whose enantiomeric purity cannot be demonstrated using the method described. In addition, Garner's aldehyde is an expensive starting material, currently costing about $100 per g.